TNF-related apoptosis-inducing ligand (TRAIL), a member of the TNF superfamily, has a strong apoptosis-inducing activity against cancer cells (Wiley, S. R., et al. 1995. Immunity 3:673-682). Unlike other death-inducing ligands of the TNF superfamily such as TNF-α and Fas ligand, TRAIL has been of particular interest in the development of cancer therapeutics as it preferentially induces apoptosis of tumor cells, having little or no effect on normal cells (Walczak, H., et al. 1999. Nat Med 5:157-163). At least five receptors for TRAIL have been identified, two of which, DR4 (TRAIL-R1) and DR5 (TRAIL-R2), are capable of transducing the apoptosis signal (Walczak, H., et al. 1997. Embo J 16:5386-5397; Pan, G., et al. 1997. Science 276:111-113; Chaudhary, P. M., et al. 1997. Immunity 7:821-830) whereas the other three (TRAIL-R3, -R4 and OPG) serve as decoy receptors to block TRAIL-mediated apoptosis (Pan, G., et al. 1997. Science 277:815-818; Marsters, S. A., et al. 1997. Curr Biol 7:1003-1006; Emery, J. G., et al. 1998. J Biol Chem 273:14363-14367). Like Fas and TNFR1, the intracellular segments of both DR4 and DR5 contain a death domain and transduce an apoptosis signal through a FADD- and caspase 8-dependent pathway (Walczak, H., et al. 1997. Embo J 16:5386-5397; Chaudhary, P. M., et al. 1997. Immunity 7:821-830; Kuang, A. A., et al. 2000. J Biol Chem 275:25065-25068). Administration of the recombinant soluble form of TRAIL in experimental animals, including mice and primates, induces significant tumor regression without systemic toxicity (Walczak, H., et al. 1999. Nat Med 5:157-163). However, as TRAIL has been shown to elicit side effects such as liver toxicity in humans, other agonists of TRAIL receptors have been developed.
Selective targeting of DR5 with a unique agonistic monoclonal anti-DR5 antibody, TRA-8, and its humanized or human versions thereof can effectively and selectively induce apoptosis of tumor cells. All TRAIL-sensitive cancer cells have been found to be susceptible to TRA-8-mediated apoptosis. Chemotherapeutic agents can synergistically enhance TRAIL-mediated apoptosis of tumor cells both in vitro and in vivo. For example, the combination therapy of TRA-8 with Adriamycin resulted in a significantly higher complete tumor regression rate than either agent alone (Buchsbaum, D. J., et al. 2003. Clin Cancer Res 9:3731-3741). These results suggest that chemotherapeutic agents might regulate the signal transduction of DR5 or the threshold of signaling required to induce apoptosis. TRA-8 has been selected as a candidate for development as a cancer therapy based on its efficacy and safety. Pre-clinical studies indicate that TRA-8 has a very strong anti-cancer efficacy in xenograft models of human cancer, particularly in combination with chemotherapy (Buchsbaum, D. J., et al. 2003. Clin Cancer Res 9:3731-3741). There is further indication that monkeys tolerate systemic administration of TRA-8 well. The binding of TRA-8 to monkey DR5 is similar to that of human DR5, and the monkeys tolerated doses as high as 48 mg/kg dose.
The expression of a death receptor by a target cell, however, is not necessarily sufficient to make the cell susceptible to the induction of apoptois by a ligand for the receptor. As an example, although most cancer cells express high levels of DR5, they are not necessarily susceptible to apoptosis induced by TRA-8, which is specific for DR5 and does not react with the decoy receptors. Furthermore, target cells such as cancer cells can show resistance to TRA-8 or other agents that induce apoptosis through death receptors (e.g., DR4 or DR5). Needed in the art is a biomarker to predict resistance and a means of reducing resistance of target cells to agonists of death receptors such as TRA-8.